430 



SURFACE VESSEL TARGET STRENGTHS 



range as the destroyer echoes, and, Hke them, showed 

 no significant dependence on speed. Differences as 

 great as 10 db were observed Ijetween the wake 

 echoes and the destroyer echoes immediately pre- 

 ceding them, but these differences appeared to be 

 quite random and unsystematic. This equivalency 

 between wake echoes and destroyer echoes is con- 

 sistent with the theory that both arise from scatter- 

 ing by small bubbles. 



The dependence of target strength on range at off- 

 beam aspects from the San Diego results shown in 

 Figure 8 cannot be analyzed very simply. In the first 

 place, the spread of aspect angles covered is very 

 wide; the projected length of the destroyer, measured 

 in a direction perpendicular to the sound beam, 

 varied from 30 ft at bow and stern aspects to 290 ft 

 at an aspect angle 20 degrees from the beam. In the 

 second place, with a pulse length of 10 msec, the en- 

 tire destroyer was not in the sound beam at the same 

 time, especially at bow and stern aspects; for aspects 

 close to the beam, this effect of pulse length may be 

 neglected, but it becomes important at other aspects. 

 When more of the target comes into the sound beam, 

 the observed echo from a very short pulse will not be 

 stronger but instead will last longer, as pointed out 

 in Section 19.3. Thus the change of target strength 

 with range at off-beam aspects cannot be explained 

 even in part by this simple mechanism. 



Transducer directivity is also relatively unim- 

 portant in the New York measurements on still and 

 moving vessels since a very wide beam was employed. 

 The total horizontal beam width of the combined 

 projector-hydrophone directivity pattern, between 

 points where the response was 10 db lower than on 

 its axis, was about 40 degrees, which even at a range 

 of 168 yd, the shortest range at which measurements 

 during either test were made, still covers the long- 

 est ship at beam aspect. Therefore the decrease in 

 target strength with decreasing range in the New 

 York measurements cannot be explained as a result 

 of the failure of the sound beam to cover the target. 



24.4.2 Predicted Dependence 



The second explanation which might be suggested 

 for the observed dependence of target strength on 

 range is the predicted decrease of specular reflection 

 with decreasing range for ranges less than the maxi- 

 mum radius of curvature of the target, providing the 

 reflection is specular (see Section 20.4.4). This 

 effect would apply only to echoes from vessels sta- 



tionary in the water, which presumably arise prima- 

 rily from the hull and not from a uniformly scatter- 

 ing layer. However, in the most extreme case, reflec- 

 tion from an infinite plane surface, the target strength 

 will not vary more rapidly than as the square of the 

 range. Such a variation is quite insufficient to ac- 

 count for the large effect observed during echo- 

 ranging trials on still vessels illustrated in Figures 5 

 and 6. Qualitatively, however, it partly explains the 

 difference in range dependence at beam and off-beam 

 aspects, since the radius of curvature of the ship is 

 greater when it presents its broadside to the incident 

 sound than when it is at bow or stern aspect. 



24.4.3 



Transmission Loss 



A third possible explanation of the observed range 

 dependence is a possible incorrect evaluation of the 

 transmission loss. In none of the measurements was 

 the transmission loss measured directly. Instead, an 

 attempt was made to estimate it from the prevailing 

 conditions on the basis of inverse-square divergence 

 and an additional attenuation proportional to the 

 range. 



At San Diego, it was assumed that the intensity of 

 the echo was inversely proportional to the fourth 

 power of the range, weakened by an additional loss 

 of 5 db per kyd of sound travel. The water was 

 isothermal to a depth of 50 ft, so that an assumption 

 of 5 db per kyd for the attenuation coefficient seems 

 somewhat low. Use of equation (1) in Chapter 23 

 gives an attenuation coefficient of about 7 db per 

 kyd. An attenuation coefficient of about 10 db per 

 kyd would be required to explain the departure of 

 the plotted points from the theoretical curve in Fig- 

 ure 7. Such a high coefficient does not seem very 

 likely when the surface layer is isothermal down to 

 a depth of 50 ft, but it is not impossible. 



At New York, the transmission loss was assumed 

 to follow the same inverse square loss with an atten- 

 uation coefficient at 27 kc of 7 db per kyd. The tem- 

 perature conditions of the water were not known; 

 the wind velocity varied from 1 to 23 mph. Whether 

 or not the assumed attenuation coefficient is reliable 

 it is difficult to say. In addition, bottom-reflected 

 sound may have had a marked effect on the trans- 

 mission loss. 



Conditions were very favorable to bottom reflec- 

 tion during these New York tests. The bottom was 

 composed of sand and mud, a mixture which reflects 

 sound very effectively. In addition, the water was 



